The present invention relates to a tuner for digital and analog use, for selectively receiving digital modulated television signals and analog modulated television signals.
Recently, with the shift to digital television broadcasting, development of television receivers capable of receiving both analog and digital broadcasts has been pursued. In the United States, and other industrial nations, the digital modulation method which has been adopted for terrestrial broadcast television is 8VSB (8 Vestigial Sideband). Since 8VSB uses the same CH (channel) bandwidth as conventional analog broadcasting (NTSC: National Television System Committee), the conventional front end can be used in almost unchanged form.
FIG. 9 shows one example of a front-end system (from front end to demodulation circuits) which uses the double-conversion method, including a conventional tuner for digital and analog use. This conventional front-end system is made up of a frequency conversion section 100, a switch section 101, a down-converter section 102, an A/D conversion section 103, a VSB demodulation section 104, and an NTSC demodulation section 105. Further, in this conventional example, members from a signal input terminal 115, through the frequency conversion section 100, up to an IF (Intermediate Frequency) signal output terminal 130 are collectively referred to as a xe2x80x9ctuner for digital and analog use.xe2x80x9d
The frequency conversion section 100 can be further divided into first and second conversion sections 106 and 107. The first conversion section 106 is made up of an RF amplifier 108, a first mixer 109, a BPF (Band-Pass Filter) 110, a first IF amplifier 111, a BPF 112, a first local oscillator 113, and a first PLL (Phase Locked Loop) circuit 114. Further, the first local oscillator 113 and the first PLL circuit 114 form a closed loop.
An input signal from the signal input terminal 115 first undergoes wideband amplification by the RF amplifier 108. Meanwhile, first PLL control data 116 is applied, via a terminal 117, to the first PLL circuit 114, and, by means of a signal from the first PLL circuit 114, the first local oscillator 113 produces a first local signal of a frequency specified by the first PLL control data 116. Then, in the first mixer 109, the output from the RF amplifier 108 and the first local signal produced by the first PLL circuit 114 and the first local oscillator 113 are mixed and up-converted into a first IF signal. The first IF signal, after undergoing band limiting in the BPF 110, is amplified by the first IF amplifier 111, and is then band limited by the BPF 112 and outputted to the second conversion section 107.
The second conversion section 107 is made up of a second mixer 118, a second IF amplifier 119, a BPF 120, a second local oscillator 121, and a second PLL circuit 122. Further, the second local oscillator 121 and the second PLL circuit 122 form a closed loop.
Second PLL control data 123 is applied, via a terminal 124, to the second PLL circuit 122, and, by means of a signal from the second PLL circuit 122, the second local oscillator 121 produces a second local signal of a frequency specified by the second PLL control data 123. Then, in the second mixer 118, the output from the BPF 112 of the first conversion section 106 and the second local signal produced by the second PLL circuit 122 and the second local oscillator 121 are mixed and down-converted into a second IF signal. The second IF signal is amplified by the second IF amplifier 119, and is then band limited by the BPF 120 and outputted to the IF signal output terminal 130.
The switch section 101, based on an external control signal 125 in accordance with the type of received signal (analog/digital), switches the destination to which to send the second IF signal from the IF signal output terminal 130. By means of the switching of the switch section 101, a second IF signal corresponding to a digital broadcast signal is sent, via a terminal 141, to the down-converter section 102. A second IF signal corresponding to an analog broadcast signal, on the other hand, is sent, via a terminal 142, directly to the NTSC demodulation section 105.
The down-converter section 102 is a signal conversion means which converts the second IF signal obtained via the switch section 101 (hereinafter referred to simply as the xe2x80x9cIF signalxe2x80x9d) to a signal suitable for input to a digital processing system. The down-converter section 102 is made up of a BPF 126, a digital IF amplifier 127, a down-converter mixer 128, and a down-converter local oscillator 129.
An IF signal corresponding to a digital broadcast signal, obtained through the switch section 101, undergoes band limiting necessary for digital demodulation, in the BPF 126, and is then amplified by the digital IF amplifier 127. In the down-converter mixer 128, the IF signal from the digital IF amplifier 127 and a signal produced by the down-converter local oscillator 129 are mixed and down-converted. The down-converted IF signal is then outputted, as a Low IF signal, to the A/D conversion section 103.
The A/D conversion section 103 converts the Low IF signal from the down-converter section 102 into a digital signal. The VSB (Vestigial Sideband) demodulation section 104 performs VSB demodulation of the digital signal from the A/D conversion section 103, and outputs a transport stream signal.
The NTSC demodulation section 105, on the other hand, performs NTSC demodulation of an IF signal corresponding to an analog broadcast signal, obtained through the switch section 101.
However, in the foregoing conventional front-end system, the frequency conversion section 100, the signal input terminal 115, and the IF signal output terminal 130 were contained, as a tuner for digital and analog use, in a single body 131, and the other circuits (the down-converter section 102, the A/D conversion section 103, the VSB demodulation section 104, and the NTSC demodulation section 105) were provided together on a common board.
Further, in the foregoing front-end system with conventional tuner for digital and analog use, a single control signal 125 was used to control the operations of the switch section 101, the down-converter section 102, the A/D conversion section 103, the VSB demodulation section 104, and the NTSC demodulation section 105.
Accordingly, with the foregoing conventional art, when receiving a digital broadcast signal, the down-converter section 102 was susceptible to the influence of noise produced by the digital processing system following the A/D conversion section 103. This caused impairment of characteristics of the down-converter local oscillator 129 of the down-converter section 102, such as precision of the local oscillating frequency. Further, since the down-converter section 102 was not enclosed by a body, there were problems with unnecessary radiation produced thereby.
Moreover, the foregoing problems also arose in a conventional front-end system for digital and analog use in which a baseband demodulation section was provided as the foregoing signal conversion means. The following will explain this conventional front-end system. Here, members having the same structure and functions as those explained above will be given the same reference numbers, and explanation thereof will be omitted here.
As shown in FIG. 10, this conventional front-end system, which uses the double conversion method, is made up of a frequency conversion section 100, a switch section 101, a baseband demodulation section 150, an A/D conversion section 103, a VSB demodulation section 104, and an NTSC demodulation section 105. Here again, a signal input terminal 115, the frequency conversion section 100, and an IF signal output terminal 130 are collectively referred to as a xe2x80x9ctuner for digital and analog use.xe2x80x9d
The switch section 101, based on an external control signal 125 in accordance with the type of received signal (analog/digital), switches the destination to which to send the IF signal from the IF signal output terminal 130. In the present front-end system, by means of the switching of the switch section 101, an IF signal corresponding to a digital broadcast signal is sent, via a terminal 141, to the base band demodulation section 150. An IF signal corresponding to an analog broadcast signal, on the other hand, is sent, via a terminal 142, directly to the NTSC demodulation section 105.
The baseband demodulation section 150 is made up of a BPF 151, a digital IF amplifier 152, an I signal demodulation mixer 153, a Q signal demodulation mixer 154, a 90xc2x0 phase shifter 155, a baseband local oscillator 156, and a phase comparing mixer 157.
An IF signal corresponding to a digital broadcast signal that is, obtained via the switch section 101 from the tuner for digital and analog use in the previous stage, undergoes band limiting necessary for baseband demodulation, in the BPF 151. Then, after amplification in the digital IF amplifier 152, the IF signal is sent to the I signal demodulation mixer 153 and the Q signal demodulation mixer 154. A signal produced by the baseband local oscillator 156 is inputted directly into the I signal demodulation mixer 153. Accordingly, in the I signal demodulation mixer 153, this signal and the IF signal from the digital IF amplifier 152 are mixed, and an I signal is outputted. The foregoing signal produced by the baseband local oscillator 156 is inputted into the Q signal demodulation mixer 154 via the 90xc2x0 phase shifter 155. Accordingly, in the Q signal demodulation mixer 154, a signal inputted from the 90xc2x0 phase shifter and the IF signal from the digital IF amplifier 152 are mixed, and a Q signal is outputted.
Here, since the phase of the baseband local oscillator 156 and a pilot carrier included in the signal inputted from the digital amplifier 152 must match exactly, the I signal and the Q signal are inputted into the phase comparing mixer 157, which detects any phase error, and which controls the phase of the baseband local oscillator 156 on the basis of the detected result. Since only the I signal is used in VSB demodulation, the baseband demodulation section 150 outputs only the I signal to the A/D conversion section 103.
The A/D conversion section 103 converts the I signal from the baseband demodulation section 150 into a digital signal. The VSB demodulation section 104 performs VSB demodulation of the digital signal from the A/D conversion section 103, and outputs a transport stream signal. The NTSC demodulation section 105, on the other hand, performs NTSC demodulation of an IF signal corresponding to an analog broadcast signal, obtained through the switch section 101.
In this type of conventional front-end system, the frequency conversion section 100, the signal input terminal 115, and the IF signal output terminal 130 were contained, as a tuner for digital and analog use, in a single body 131, and the other circuits (the baseband demodulation section 150, the A/D conversion section 103, the VSB demodulation section 104, and the NTSC demodulation section 105) were provided together on a common board, and a single control signal 125 was used to control the operations of the switch section 101, the baseband demodulation section 150, the A/D conversion section 103, the VSB demodulation section 104, and the NTSC demodulation section 105.
Accordingly, in the same way as in the front-end system provided with the down-converter section 102 (see FIG. 9), when receiving a digital broadcast signal, the baseband demodulation section 150 was susceptible to the influence of noise produced by the digital processing system following the A/D conversion section 103. This caused impairment of characteristics of the baseband local oscillator 156 of the baseband demodulation section 150, such as precision of the local oscillating frequency. Further, since the baseband demodulation section 150 was not enclosed by a body, there were problems due to the production of unnecessary radiation.
It is an object of the present invention to provide a tuner for digital and analog use which is able to reduce the influence of noise from a digital processing system on a signal conversion section made of a down-converter section, a baseband demodulation section, etc., and which is able to reduce unnecessary radiation from the signal processing section.
In order to attain the foregoing object, a tuner for digital and analog use according to the present invention is a tuner for receiving both digital broadcast signals and analog broadcast signals, and includes: a frequency conversion section, which converts a received signal to an intermediate-frequency signal; a destination determining section, which determines a destination to which to send the intermediate-frequency signal from the frequency conversion section; a signal conversion section, which converts the intermediate-frequency signal obtained through the destination determining section to a signal suitable for input to an external digital processing system; and a body, which contains the frequency conversion section, the destination determining section, and the signal conversion section.
With the foregoing structure, since the frequency conversion section, the destination determining section, and the signal conversion section are contained in a single body, the tuner for digital and analog use and the external digital processing system can be sufficiently electrically isolated from one another. Thus, a tuner for digital and analog use can be realized which is less susceptible to noise from the digital processing system, and thus it is possible to avoid impairment of the characteristics of the signal conversion section by that noise. Further, by enclosing the signal conversion section in the body, it is isolated from the digital processing system, and thus it is possible to reduce unnecessary radiation from the signal processing section.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.